This disclosure relates generally to improved systems and methods for electro-enhancement of metal surfaces.
Electrochemical methods exist to enhance the surfaces of metal and non-metal materials, in particular ferrous metals, with one or more impregnating species, such as boron. The impregnating species can occupy positions within a crystal structure of the metal or non-metal material or can form compounds with the metal or non-metal material. Thermochemical processes utilize mixtures of compounds and reactants that, when heated, release impregnating material vapors that interact with metal or non-metal materials at high temperatures. The mixture can be packed around parts or locally applied as a paste. One example of such a process is pack boriding. Processes like these are widely applied due to the simplicity and low capital investment, but downsides include the labor-intensive nature of the process and a significant waste stream, and the generation of boron and its reaction with and diffusion into the substrate are not independently controllable, limiting the range of outcomes.
Other methods exist, including gas-based or plasma-based methods where impregnating materials can be introduced into heated chambers, high-energy ion beam implantation, or intense plasma treatment, but each of these processes has shortcomings.
In the context of boronizing of ferrous metals, and in particular steel, two types of boron compounds are commonly formed on the surface of the ferrous metals. At the beginning of the process, Fe2B is the dominant species. Fe2B is a hard compound that provides excellent wear properties for use in tool applications. Fe2B has thermal expansion characteristics similar to steel, and its icicle-like protrusions into a steel substrate that strengthen the binding between the Fe2B and the steel substrate. Fe2B can withstand mild flexure and thermal shock without flaking off the surface. However, as the Fe2B forms, the resistance to further flow of boron into the steel increases. A simple analogy is that of a blotter or paper towel edge contacting water, where after an initial inflow, the process slows due to the “back pressure” from the wetted portion.
When a boron concentration near the steel exceeds a certain threshold that is necessary for the inward growth of the Fe2B, FeB forms on the surface. FeB is harder than Fe2B, but much more brittle. Moreover, FeB has thermal expansion properties that are much different from steel, so the FeB tends to chip off (known as spallation) when parts are cooled in post-processing. It is known that FeB can be reduced or eliminated by subsequent heat treatment in the absence of current, boron, and oxygen. However, this means of reducing or eliminating FeB can result in a porous material with less than ideal material properties.
Thus, a need exists for systems and methods for improved electro-enhancement of surfaces that overcome the shortcomings discussed above.